It depends on what type of infection we are worried about and what structures are involved but generally an MRI and a lumbar puncture for cerebrospinal fluid an

It depends on what type of infection we are worried about and what structures are involved but generally an MRI and a lumbar puncture for cerebrospinal fluid analysis to start. If there's an abscess a neurosurgeon can stick a needle/drain in it. Usually a combination of clinical and MRI findings is enough for infectious diseases and neurology specialists to figure it out and start empiric treatment for something/a few somethings if the CSF doesn't give you the answer. Rarely, at least at my institution, you could do a biopsy but you don't really want to be chopping up the brain if you can avoid it.